Science1 publisher3 min readPublished
A Costa Rica expedition's 371 sea snakes carried crabs, shrimp and a clingfish
Joseph Pfaller noticed a crab drop off almost the first yellow-bellied sea snake he handled in 2012, and the larvae he collected then waited about a decade in a Florida museum for a crab specialist to ask what they were.
The Scientist · Science desk
What happened
- In 2012 Joseph Pfaller joined a University of Florida expedition off Costa Rica led by Harvey Lillywhite, and a small crab fell off almost the first yellow-bellied sea snake the team caught.
- The expedition worked through 371 sea snakes, which hosted occasional crabs, shrimp and snails, and even a small clingfish.
- Nothing with legs had ever been reported living on a sea snake; the known epibionts were barnacles, tube-dwelling worms and other animals that stay put.
- A single juvenile crab in the genus Planes, a group Pfaller knew from sea turtle shells, gave him a candidate name for the unidentified larvae in his jars.
- Pfaller deposited the specimens at the Florida Museum of Natural History and moved on to a sea turtle dissertation, leaving dozens of megalopae unidentified.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability Sequencing can put a name on a crab stage that morphology cannot settle. Larval material already sitting in museum jars becomes identifiable years after collection.
- constraint A collection this size establishes that the riders are there and leaves open how often, so no one can yet say whether a sea snake is habitat for these crabs or a perch the odd one finds.
- precedent If riders this visible went unrecorded on a snake as closely studied as Hydrophis platurus, other open-ocean hosts are probably carrying epibionts nobody has looked for.
A yellow-bellied sea snake at the surface has no rock or reef to rub against, so it wriggles into a knot and works its old skin off with the friction of its own scales [1]. "When you find a sea snake's skin, it's usually in a knot," Pfaller said [2]. When the crab dropped off the snake in his hands, his first question was whether it had been living there until the snake shed [20].
Whether the crab had been living there until the snake shed is a question about timing. Identity is what the paper takes on, under the title "Planes on a Snake? On the Identities of Crab Larvae Rafting on Sea Snakes" in Ecology and Evolution [6]. Testing the molt idea takes a different design: each crab scored by developmental stage against the shed stage of the snake it came off, with enough snakes at each stage to compare. Pfaller's account describes the eviction as a question he had.
A collection like this settles presence; frequency is a different claim. The riders were described as occasional [10]. A prevalence figure would need the empty snakes counted as carefully as the occupied ones, and the numbers here are counts of animals found.
A megalopa is genuinely hard to identify: the stages of a crab's life cycle can look radically different from one another, little has been published on the anatomical diversity of megalopae, and Pfaller could not name his in 2012 [11]. "I knew Planes from my turtle work, so I thought the megalopae might be the larval stage of Planes," he said [13]. Naming them accurately required sequencing DNA [18]. Phys.org headlines the result as various crab species hitching rides on sea snakes, so more than one kind is in the collection [24].
Sedentary animals were all anyone had reported from sea snakes. Darwin wrote up an unknown barnacle a colleague sent him from a snake near Borneo [7]. In 1975, two types of tube-dwelling worm and a moss animal came off a Dubois' sea snake, and the scientists who found them suggested something odd about that snake's behavior had attracted such unlikely freeloaders [8]. Thirty-seven years separate that report from the Costa Rica expedition [21]. Occasional crabs, shrimp and snails spread across hundreds of snakes are hard to pin on one animal behaving strangely.
Around 2022, by then a courtesy faculty member at the University of Florida, Pfaller mentioned the shelved project to Robert Lasley, a crab expert working as a postdoctoral researcher at the Florida Museum [15][22]. Lasley cut him off, as Pfaller recounted it: "Wait! Wait! Wait! Are you saying you have Planes on a snake?" [16] "That's when we decided, all right, no matter what happens, we have to do this," Pfaller said [17].
What to watch
- Whether the sequences place the megalopae in Planes alone or spread them across several crab genera.
- Whether a follow-up counts crabs per snake and scores each snake's molt stage. A claim about molt timing needs both.
- Whether preserved collections of other sea snake species yield legged epibionts when someone re-examines them.